1. Field of the Invention
The present invention relates to a lens unit through which light passes, and an image taking apparatus which obtains image data by forming an image of a subject.
2. Description of the Related Art
Some of electronic still cameras which form an image of a subject on a solid imaging element such as a charge coupled device (CCD) and which captures image data representing the subject as a signal, and some of film cameras which shoot a subject on a photographic film include zoom function for freely setting shooting angle of view. Such a camera includes a shooting lens whose focal length is varied in accordance with operation of a zoom switch. This shooting lens generally is a compound lens comprising a combination of lens elements, and relative positions of the lens elements are adjusted in accordance with focal length which is set by the zoom switch. Such a camera includes a cam mechanism, the cam mechanism transmits rotation of a motor in accordance with operation of the zoom switch, thereby moving the lens elements forward and backward in an optical axis to adjust the relative positions, and the focal length is changed.
A focus lens for adjusting focus is included in the lens elements. A lens driving mechanism for moving the focus lens is disposed independently from the cam mechanism in some cases.
As an alternative lens of the shooting lens having the driving mechanism, it is recently proposed a liquid lens having variable focal length in which two kinds of mutually non-mixable liquids having different refractive indices are accommodated (see non-patent document “Philips Fluid Lenses”, [online], dated Mar. 3, 2004, by Royal Philips Electronics, [Searched on Mar. 31, 2004], the Internet <URL: http://www.dpreview.com/news/0403/04030302philipsfluidlens.asp> for example).
The two kinds of mutually non-mixable liquids having different refractive indices are accommodated in the liquid lens proposed in the non-patent document. One of the two kinds of liquids is conductive aqueous solution, and the other liquid is insulative oil. The two kinds of liquids are accommodated in a short glass tube liquid container. Both ends of the liquid container are closed with light-transmitting clear endcaps. An inner wall of this tube and an inner wall of one of the endcaps are covered with repellency films. According to the liquid lens having such a structure, the conductive aqueous solution of the two kinds of liquids is repulsed from the inner wall of the tube covered with the repellency film and from the inner wall of the one endcap. Since the conductive aqueous solution stays in a form of a semi-spherical shape in a state in which the conductive aqueous solution is in contact with the other endcap, an interface portion between the conductive aqueous solution and the insulative oil functions as a concave lens. The liquid lens is also provided with two electrodes for applying voltage to the conductive aqueous solution. One of the two electrodes is disposed in contact with the conductive aqueous solution, and the other electrode is disposed behind the repellency film. If voltage is applied to such electrodes, the electrode disposed in contact with the conductive aqueous solution discharges electric charge into the conductive aqueous solution, and the discharged electric charge is accumulated in the interface portion between the conductive aqueous solution and the insulative oil therein. The electric charge accumulated in the interface portion and electric charge collected in the electrode disposed behind the repellency film having opposite pole from that of the former electric charge attract each other due to the Coulomb force, and the electric charge in the conductive aqueous solution is attracted close to the repellency film. As a result, the conductive aqueous solution starts wetting the repellency film which covers the inner wall of the tube, and the interface shapes of the two kinds of liquids are changed. That is, as the voltage is strongly applied to the conductive aqueous solution, the radius of curvature of the interface portion of the conductive aqueous solution between the insulative oil and the conductive aqueous solution which first functioned as the concave lens is changed, and for example, the interface portion becomes completely flattened or the conductive aqueous solution functions as a convex lens, and the focal length is changed.
According to such a liquid lens, since the focal length can be changed without moving the lens, it is possible to realize zoom function and focus function without providing the cam mechanism or the lens driving mechanism. Therefore, the device can be made compact, and the device can be applied to a small device such as a mobile phone.
According to the liquid lens described in the non-patent document, even if voltage is applied and a target shape of the interface between the liquids can be obtained, since gravity is applied, the interface shape is distorted. As a result, there is a problem that the focus position of the liquid lens is deviated, and image deviation is generated in a shooting image.